"He had a phenomenal game last week (against South Africa). I think he is a sensational player," Gatland said of the 32-year-old Blues forward, who made his first Wales appearance in 2002 and has featured in five British and Irish Lions Tests.

"He has dropped four or five kgs and he is looking really fit at the moment. He has been training hard.

"For me, it is a fantastic milestone for someone who has been a great servant of Welsh rugby.

"I hope we can go out there on Saturday and give a performance that reflects his contribution to Welsh rugby in that we play well and win."

Gatland has made four switches from the side beaten 24-15 by South Africa, with Allen replacing an injured Jonathan Davies alongside Scott Williams in midfield and Biggar starting instead of Rhys Priestland.

In the pack, Scarlets front-row prospect Rhodri Jones takes over from Adam Jones, who has a calf muscle problem, and an ankle injury sidelines flanker Dan Lydiate, so Justin Tipuric wears the number seven shirt and skipper Sam Warburton moves to blindside.

Rhodri Jones, 21, first featured for Wales against the Barbarians last year, and he was also part of the 2013 summer tour to Japan. He has won two caps, but was only called up by Gatland last Sunday as squad injury cover.

Biggar, meanwhile, was the Wales number 10 throughout last season's title-winning RBS 6 Nations campaign, taking his opportunities well while Priestland was sidelined with an Achilles tendon injury.

And the highly-rated Allen, who cut his representative teeth on the international sevens circuit with Wales, has a golden chance to impress Gatland building towards the autumn finale against Australia on November 30 and further ahead to World Cup 2015.

"With Jonathan being injured we are looking for some cover in the centres. It is a chance for a young guy to develop at centre and get some strength in depth there," Gatland added.

"The number 10 call was a tough call last week. The plan was to play one in the first game and one in the second game, and we will probably look at James Hook next week (against Tonga), then make a decision on the Australia game.

"With Adam Jones being injured we've had Rhodri in with us. He has been going well and we've been impressed with him, and with Dan (Lydiate) injured we have spoken to Sam about playing six. It is the same back row that played against England in the Six Nations."

Wales have not won an autumn international since they beat Argentina 33-16 four years ago, while the Pumas prevailed 12 months ago during a demoralising November series that produced four home losses.

"The challenge on Saturday is obvious from the disappointment of last year," Gatland said.

"We have been working this week in terms of some support lines and making sure that when we make a break our lines of support are a bit better than they were against South Africa.

"It is just encouraging the players to have the ability to play what is in front of them. We give them the structure, but that doesn't mean we are inhibiting them in any way.

"For me, the most exciting part of the game last week was Rhys Priestland running the ball from his own goal-line and having the vision to see that and move the ball. We want to encourage the players to do that and have that confidence to go out there and express themselves.

"We've had a hit-out now, and we need to improve from last weekend, finishing off the chances we create.

"When Jonathan Davies went through, if you look at the lines of support they were not positive enough. The top teams finish it off when they make a break because their support lines are so positive and they get on the inside of the pass.

"We are working on the fine margins now to try to get this team to that top level. Those little one per-centers are now a huge priority for us."